A long-tailed weasel pokes its head out of a burrow. The predator, whose brown coat turns to camouflaging white in winter, prowls the subnivean world looking for voles, mice, and shrews. Photo: Photograph by Michael Quinton/Minden Pictures/National Geographic Stock

Snow is the most transforming element. It changes everything. Winter’s first covered landscape seems a different place than the world just a few hours earlier—softer, quieter, held more still. Its whiteness glints clean, sparkling in bright sunlight like a million tiny diamonds brought to a boil. Later, blue shadows pooling in subtle hollows enforce the impression of coolness.

But snow affects more than just the way the world looks. It alters the lives of creatures living in and beneath it. And on a level that is at once minute and massive, its arrival triggers a myriad of lifecycles that, until the past decade, nobody knew existed. The role of microbial life beneath the snow, it turns out, may have serious ramifications for how we measure and gauge the effects of global warming.

. We found microorganisms were growing and holding on to the nitrogen, not just from the snowpack but also the nitrogen that was in the soil before snow started to fall. It was one of those surprising things where you think, ‘Hey, wow! I wonder what’s going on here?’ ”

To find out, they traveled to New Mexico, Colorado, Wyoming, and Alaska, taking samples in a wide range of altitudes and ecosystems (coniferous and deciduous forests, meadows, treeless alpine slopes). Brooks found—and others confirmed—that winter is a time of great biological activity that produces massive amounts of carbon dioxide.

Brooks’s latest data indicate that how much carbon microbes release is closely tied to when snow falls. A cold snap that occurs before a blanket of snow insulates the ground will freeze at least part of the plant and microbial life that has been growing all spring and summer. When plants and other organisms freeze, ice crystals can rupture cells and break down the tissues, which microbes and other scavengers that remain unfrozen can then use as food when they thaw.

Brooks found that when all other conditions remain similar, a delay in snowfall creates a huge difference in the amount of CO2 respired into the atmosphere. To a point, the more severe the freeze, the more dead plants and microorganisms are frozen—and the more ice crystals rend open tissues. As a result, more microbe colonies may thrive when it warms under the snow cover, yielding 25 percent to 200 percent more CO2 into the air. “Up to half the carbon that plants take up in summer is released back into the atmosphere by microbes in winter,” Brooks says, a fact nobody knew until a few years ago. This means estimates of how much carbon seasonally snow-covered forests absorb must be recalibrated to account for the carbon collectively released by the respiration of billions of tiny creatures. If a warmer world means the delayed arrival of the snowpack, even more carbon could be released in the future.

There is a tipping point. If snowfall arrives too late, the ground remains bare and frozen too long, and ice sublimates into the air. Think of freezer burn: Leave a piece of fruit in a plastic bag in the freezer too long and ice crystals form inside the bag. Water that was in the fruit has sublimated, then condensed on the bag, forming ice. The same thing happens in bare soil, only there’s no bag to trap the water vapor, which dissipates into the air. The ground grows dry. As a result, though there’s plenty of food for microbes, without water their growth is retarded.

Fewer, smaller microbe colonies means less carbon respired into the atmosphere—which would seem like a boon for the climate except that it has another serious repercussion. Subnivean microbes, it turns out, provide much of the nitrogen that plants need to grow in the spring. They absorb nitrogen from the snow and from decomposing plants in the soil. As snow melts in the spring and these organisms die, nitrogen is freed at precisely the time plants emerging from winter need it to grow. The microbes provide a critical nitrogen banking service for vegetation in seasonally snow-covered ecosystems.

There’s more: If a lack of nitrogen stunts vegetative growth, the amount of carbon that grasses, shrubs, and trees can remove from the atmosphere in spring and summer is also diminished. Essentially, that means less climate-change-causing carbon is pulled from the atmosphere and stored in plants. “There seems to be a very, very delicate balance,” Brooks says, “a situation where a little change in just the timing of the snowpack could cause tremendous changes in how these systems function, both summer and winter.”

Animals like voles and deer mice survive under the snow at the mercy of conditions. Their success in any given year drives ecosystem health. Populations of animals from snowy owls to grizzly bears are, in part, regulated by the number of rodents that survive beneath the snow. The more voles, shrews, and mice there are, the more predators there will be.

The smaller, microbial life that flourishes under the snow is just beginning to be understood, and it, too, prospers according to the whims of weather. But the lifecycles of subnivean microbes seem to have even further-reaching ramifications than local population dynamics. We see snow as a transformative element, brilliantly whitening our vision. But beneath its still surface, these tiniest of life-forms ebb and surge, ravaging secret winter feasts. Their tiny subnivean lives and deaths alter the composition of our atmosphere—and may affect the entire planet’s health.

This story originally ran in the November-December 2012 issue as "Packed to the Hilt."